In the last years, there has been a process of change in the requirements and specifications for low and medium voltage cables for works and installations in the building industry that, because of their characteristics and uses, require a high fire safety level. This process started when, after intensive studies, it was observed that a large number of fire-related deaths, either in houses, buildings or industrial installations were caused by smoke emissions generated by the combustion of products such as electrical conductor insulation materials. With regard to the importance of this problem, statistics show that over 80% of the victims of fires die because of smoke intoxication.
Commonly, polyvinyl chloride (PVC)-based plastic compounds have been used as insulation material for low voltage electrical conductors because of their low cost and excellent flame-retardant effect that can be enhanced by additives, among them polybrominated compounds such as the ones described in patents WO/1989/003409 and U.S. Pat. No. 4,892,683A1, antimony trioxide, as described in U.S. Pat. No. 5,863,963, or even metal hydroxides such as the ones described in U.S. Pat. No. 6,087,428A1. However, because it is a resin that includes at least one halogen such as chlorine in its chemical structure, it has some undesirable characteristics when burned, i.e., the generation of dark and dense gases, as well as, the release of hydrochloric acid. Moreover, the presence of chlorine in its molecule limits its heat resistance to a service temperature not higher than 105° C.
The above has motivated the search for the development of halogen-free alternative materials. In this sense, efforts have been made to produce materials emitting low smoke levels as described in U.S. Pat. No. 5,912,436A. Despite all these efforts, the problem has not been solved satisfactorily. There is still a need for developing cables with insulations showing a low generation of smoke and toxic gases during combustion, maintaining good electrical thermal insulation properties, as well as, a good flame retardant behavior. However, faced with the continuous need of reducing combustion toxic gases at the lowest possible levels, it is necessary to take into account the use of other polymeric materials allowing a better performance.
Thus, polyolefins (HDPE, LLDPE, PP, etc.) are a good option to substitute PVC, because, compared to it, they show lesser smoke emission, better electrical properties and are more environment-friendly. On the other hand, polyolefins show limited UV light resistance, a low resistance to oxidation and, because of their nature, a low flame resistance and thus require the use of large quantities of mineral fillers to improve their performance. The problem becomes critical if we take into account that most of the polyolefins have a limited capacity to accept mineral fillers and when their content is increased within the polyolefin, a reduction of mechanical properties is observed.
Among polymers showing low smoke generation during combustion, polyolefins are to be mentioned. Polyolefins are polymers that are less reactive because they are formed by saturated carbohydrate and because their macromolecules are not chemically bound together, except in crosslinked products. Because of this characteristic, the polyolefin polymers that have had a good application in the insulation of electrical conductors are polyethylene, as described in US Patent Publication 20080300370 and WO2008147494; the homopolymer polypropylene described in patents U.S. Pat. No. 4,522,994 and U.S. Pat. No. 6,919,410B2; the copolymer described in US Patent Publication 20090326157A1; patents JP 4-261413A and U.S. Pat. No. 4,424,330. On the other hand, the disadvantages shown by the polyolefins as pure resins are lack of flame resistance and, in most of them, a low capacity to accept mineral fillers that give them flame resistance property. Another problem that needs to be taken into account when working with polyolefins is their high sensitivity to oxidative degradation caused by ultraviolet light (UV) and the oxygen, which becomes more obvious in the case of polypropylene and its copolymers.
In order to try to solve the abovementioned problems, various additives have been used in the formulation of polyolefin-based compounds to obtain the characteristics required for processing them and for their performance as a finished product. Many other works conducted with polyolefins have been focused on integrating more than 100 parts per hundred of resin (phr) of mineral fillers, maintaining elongations at rupture above 100%, as described in U.S. Pat. No. 7,125,924B2 and US Patent Publication 20040127630, or in compounds that use coupling agents enhancing the compatibility between polymer and mineral filler as described in document WO2007/130407A1. Some other developments such as the ones described in U.S. Pat. No. 6,492,453 and U.S. Pat. No. 7,078,452B2 combine, besides the coupling agent, the use of mineral nanofillers to make flame retardant articles.
Other works have been focused on the development of flame retardant systems that can be used in smaller concentrations than metal hydroxide. Thus, systems based on acrylic polymer combinations, silicon compounds and calcium carbonate have been produced, the level of use of which are below 100 phr as described in patent EP0393959A2.
With regard to the application of polyolefin-based electrical insulation in cables, the development of flame retardant products with the addition of vinylsilane to the polyolefin composition and then crosslinked through moisture has also been explored, as described in patent EP1288970A1. In some applications, such as insulation for thin wall automotive cable and to fulfill water immersion electrical tests, new nitrogen-phosphorus-based flame retardant have been developed with good results, as well as, some tetraoxaspiro-type compounds to form fire inert material, that even meet the requirements of standards such as ISO6722, as described in patent EP1990808A3. It is very complex to obtain good electrical insulation in long term tests in moist environment and good flame resistance properties within the same compound. For this reason, some developments such as the one protected by U.S. Pat. No. 6,828,022B2 have considered the design of an electrical conductor with two insulating layers, a moisture resistant layer and a flame retardant layer.
With regard to the development of polymer formulations for use as insulations for electrical cables, the reduction of the mechanical properties is not the main problem of incorporating flame retardant mineral fillers to the polyolefins. There is a greater complexity related to the reduction of the electrical properties of the compound in cables immersed in water during several days.
In short, it is highly complex to develop a cable having a polyolefin-based, halogen-free, thermoplastic insulation compound that combines, as a finished product, the characteristics of a service thermal class −40° C. to 105° C., very low smoke emission, high flame resistance measured as FV2 vertical flame test (VW-1), high resistance to fire propagation, good electrical properties in water immersion tests during several days and good processability when applied through the extrusion process in the manufacturing of the cable.